The Omicron variant of coronavirus is now circulating within the community, the UK's health secretary has confirmed.
Sajid Javid told MPs today that "multiple regions of England" were seeing cases of the variant that were not linked to international travel.
He said he could not guarantee the variant would not "knock us off our road to recovery", as he said "the window between infection and infectiousness may be shorter for the Omicron".
Earlier, Professor Paul Hunter, from the school of medicine at the University of East Anglia, had warned the Omicron variant could be spreading faster than a previous variant, Delta.
He told BBC Breakfast: "How it's likely to spread in the UK is still uncertain, but I think the early signs are that it will probably spread quite quickly and probably start outcompeting Delta and become the dominant variant probably within the next weeks or a month or so at least.
"The big remaining question is actually how harmful it is if you do get Covid with this Omicron variant, and that's the question that we're struggling to answer at the moment."
Mr Javid told the commons there were now 261 confirmed Omicron cases in England, 71 in Scotland, and four in Wales.
He said: "We are learning more about this new variant all the time.
"Recent analysis from the UK Health Security Agency suggests that the window between infection and infectiousness may be shorter for the Omicron variant than for the Delta variant, but we don't yet have a complete picture of whether Omicron causes more severe disease or indeed how it interacts with the vaccines.
"We can't say for certain at this point whether Omicron has the potential to knock us off our road to recovery."
He added: "We are leaving nothing to chance. Our strategy is to buy ourselves times and to strengthen our defences while our world-leading scientists assess this new variant and what it means for our fight against Covid-19."
The government said, as of 9am today, there had been a further 51,459 lab-confirmed Covid-19 cases in the UK.
It also said that a further 41 people had died within 28 days of testing positive for Covid-19.
Separate figures published by the Office for National Statistics show there have now been 170,000 deaths registered in the UK where Covid-19 was mentioned on the death certificate.